A network packet is a formatted data item that is transmitted by a network. Formatting data into packets allows for packet switching and sharing of network bandwidth among a large number of users. A packet includes control information and user data, which is also known as the payload. Control information includes metadata for delivering the payload, such as source and destination network addresses, error detection codes, etc. Typically, control information is found in packet headers and/or trailers.
A service function is a specific treatment applied to data packets by a physical or virtual network device. Examples of service function include firewall, WAN, application acceleration, deep packet inspection, lawful intercept, server load balancing, etc. A service function path is a network path traversed by data packets to receive an ordered set of service functions. A service function chain is the ordered set of service functions associated with a service function path. A network service header is a data item containing control information and included in the data packets to support the service function chain. The service function, service function path, service function chain, and network service header are described in RFC (Request for Comments) 7665, an Internet Engineering Taskforce memorandum on Internet standards and protocols.